This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. The extracellular signal molecule autoinducer-2 (AI-2) mediates quorum sensing communication in diverse bacterial species. In marine vibrios, binding of AI-2 to the periplasmic receptor LuxP modulates the activity of the inner membrane sensor kinase LuxQ, transducing the AI-2 information into the cytoplasm. At low cell density, when LuxP is not bound to AI-2, LuxQ functions as a kinase, whereas at high cell density, when LuxP is bound to AI-2, LuxQ acts as a phosphatase. We have shown that Vibrio harveyi LuxP associates constitutively with LuxQ in both the presence and absence of AI-2. The 1.9 [unreadable][unreadable][unreadable] x-ray crystal structure of apoLuxP, complexed with the periplasmic domain of LuxQ, reveals that the latter contains two tandem PAS (Per/ARNT/Simple-minded) folds. Thus, although many prokaryotic PAS folds themselves bind ligands, the LuxQ periplasmic PAS folds instead bind LuxP, monitoring its AI-2 occupancy. Mutations that disrupt the apoLuxP:LuxQ interface sensitize V. harveyi to AI-2, implying that AI-2 binding causes the replacement of one set of LuxP:LuxQ contacts with another. These conformational changes switch LuxQ between two opposing enzymatic activities, each of which conveys information to the cytoplasm about the cell density of the surrounding environment. Comparison of the apo complex and holo complex (proposed here) structures will reveal AI-2-induced conformational changes, and therefore, the mechanism of AI-2 signaling regulation. Bacteria modulate gene expression in response to changes in cell density via a mechanism termed quorum sensing. This density-triggered response results in coordinated phenotypic changes within bacterial populations. Quorum sensing directs processes including virulence factor expression, biofilm development, bioluminescence, antibiotic production, and DNA exchange. Our studies will facilitate the rational design of broad-spectrum antibacterial compounds that interfere with quorum sensing. Discovery of such compounds is critical as bacteria increasingly become resistant to current antimicrobial therapies.